1. Field of the Invention
The present invention relates to an image input apparatus for forming image information of an object through photoelectric conversion.
2. Related Background Art
An image input apparatus using a linear image sensor is known.
FIG. 1 shows the structure of a linear CCD image sensor used in a conventional image input apparatus.
In FIG. 1, reference numeral 101 represents a light receiving pixel array of a linear CCD image sensor. Reference numeral 102 represents an analog shift register for sequentially reading electric charges accumulated in odd pixels of the pixel array, and reference numeral 103 represents an analog shift register for sequentially reading electric charges accumulated in even pixels of the pixel array. Reference numerals 104 and 105 represent output amplifiers for converting the electric charges read by the analog shift registers 102 and 103 into voltage signals.
Electric charges of the pixel array 101 of the linear CCD image sensor shown in FIG. 1 are read by grouping the pixels into even and odd pixels, in order to obtain a read speed higher than a predetermined speed, because the analog shift registers 102 and 103 have a limit in the transfer speed.
An image input apparatus having a read speed higher than that of a conventional linear CCD image sensor of an even/odd separation read type such as shown in FIG. 1 has been desired.
A linear CCD image sensor capable of realizing a read speed two times as fast as that of a linear CCD image sensor of an odd/even separation read type has been proposed. As shown in FIG. 3, this linear CCD image sensor has a right/left division read structure in addition to an even/odd separation read structure.
The linear CCD image sensor shown in FIG. 3 has four analog shift registers 302 to 305 to read electric charges from a light receiving pixel array by dividing them into right and left pixels and grouping each of the right and left pixels into even and odd pixels.
The linear CCD image sensor of the right/left division type shown in FIG. 3 is, however, associated with the following problem.
A level of each of the read signals of right/left four channels becomes different at the boundary between the right and left pixels if an offset of read signal of each channel has a different offset level.
In a conventional linear CCD image sensor of the even/odd separation read type, even if there is a signal level difference between even/odd pixels, only a very fine repetitive pattern appears on an image. However, in a linear CCD image sensor of the right/left division read type, even if difference between a read signal levels of the four channels is very small, a visually conspicuous line appears on an image at the right/left boundary.
Such offset levels can be corrected by proper means.
Such correction is performed once prior to reading an original, and thus the following problem occurs. An offset level fluctuates during the original read operation because of temperature change in a CCD sensor, analog circuits, video circuits and the like. Therefore, the read signal level fluctuates as shown in FIG. 2. This fluctuation causes clear difference in a signal level between the right and left images picked up with an image input apparatus using a linear CCD image sensor of the right/left division read type.